Sam Biddle

Terrorists are running out of places to put their bombs—even shoes are off limits now. So where do you stick explosives these days? Here's a look inside the deviously clever design of printer bombs that made recent headlines.

Officials aren't certain what group is behind the improvised bombs that found their way onto cargo planes this past week—but they demonstrate an ingenius and sophisticated design with basic electronics you probably have sitting around your home or office.

The idea was simple. Pentaerythritol tetranitrate, more commonly known as PETN, is a relatively stable explosive. To detonate it, you basically need to blow it up with something else—or hit it really hard. But unlike other explosive compounds, the risk of a PETN-based bomb destroying itself accidentally is low. And a little bit goes a long way—a fifth of an ounce of the stuff will blow a hole in metal twice the thickness of an airplane fuselage.

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So these as-for-now anonymous bomb makers took around 7 pounds of PETN and stuffed it—rather crudely—inside a printer cartridge, along with some equally crude electrical wiring. All of this was linked up with a SIM card just like the one you have inside your phone. The job wasn't exactly spotless, bomb experts note, but it was clever enough to keep it hidden for a disconcerting length of time. The assumption is that the SIM card would have been used to detonate the explosive printer remotely, via a phone call. But so far, there's no sign any antenna was present—so where would the bomb have received a signal? And what was going to be used to detonate this hard-to-detonate PETN? Nobody really knows yet. But the design betrays an eye for technical cunning that's at once amateurish and, morbidly, brilliant. [Danger Room]